Just bought a new pair of 700c hybrid wheelset with an inside rim width of 19mm. I have tried to fit schwalbe 700 x 40c tyres on them but the tyres just pop out from the rim when inflated. It seems to be a loose fit. Somebody please advise.

I would suggest you have the wrong size tires. I no expert but if you look on the tires or google bicycle tires there are numbers on them that tell you the exact size. Or you could take the old and the new tires into your local bike shop and they will straighten it out in a minute.

Could be a couple things.
- You could be fitting the tires wrong. If you're not getting the bead equally seated all the way around it will push off when you inflate. Try installing the tire, pumping up to around 20-30 psi, and then inspect.
- If the tube is getting caught under the bead of the tire, that can push it up an over the edge of the rim when you inflate. Try inflating the tube just enough so it holds a doughnut shape when you put it in the tire and on the rim.
- could be a bad combination of rims & tires. Even though they are all the same "size", some rims run slightly small and some tires run slightly big. Put them together and you can have trouble. You could try another brand tire just to see. I've never experienced Schawlbes to be particularly loose, but all brands vary a little.
- Could be just too wide a tire for those rims.

You might want to go into a shop and have someone pro try installing to help determine if it's something you're doing or if there's a problem with the parts.